The application of coherent optical devices operating in the region of the infrared spectrum from about 3 to about 5 micrometers has proven useful in environmental monitoring. These instruments, in which a laser generates intense infrared pulses in beam widths as small as 30 seconds of arc, measure concentration of gaseous molecules in the earth's atmosphere by radar techniques. Unfortunately these lasers are not tunable over broad ranges.
An optical parametric oscillator (OPO) using an optically nonlinear lithium niobate crystal has proven to be practical for tuning the wavelength of the coherent optical instrument. The lithium niobate crystal in the OPO is employed as the interaction medium for tuning and is pumped by a laser operating substantially at 1.064 micrometers in the infrared in a direction substantially normal to the optic axis of the crystal to produce infrared light in the 3-5 micrometer range. The wavelength is turned by adjusting the mechanical angle theta .theta., at which the incoming laser light strikes the crystal, by rotating the crystal about an appropriate axis. Tuning of the wavelength can also be achieved by varying the temperature of the OPO crystal. It should be appreciated by those skilled in the art, however, that in a singly resonant oscillator only one tuning technique is desired.
Accordingly, it is an object of this invention to precisely control the temperature of the OPO crystal so that tunable oscillations in the optically generated wavelength can be provided by rotating the crystal about an appropriate axis.
Another object of this invention is to provide a coherent optical device having minimal error resulting from temperature variation.
Another object of this invention is to provide a temperature controller for a crystal mounted in a rotatable support which anticipates correction of an error by sensing the rate of change of the error and providing a corresponding correction.